1. Field of the Invention
This invention constitutes an improvement to the dryer section of a modern papermaking machine. The improvement involves the incorporation into the dryer section of components normally found only in the press section More specifically, the dryer section, modified according to this invention, would include press rolls adjacent to the dryer cylinders; wet press felts rather than the usual dryer felts; and pocket ventilation rolls to dry the wet press felts after absorbing water from the paper sheet.
2. Description of the Prior Art
Modern papermaking machines are typically composed of these sections: the forming, press and dryer sections. All three sections share the common goal of removing water from the material which eventually emerges from the end of the machine as paper. The process begins in the forming section, where the furnish, an aqueous suspension of wood fiber, is deposited on the forming wire, a moving, screen-like continuous belt. Also known as a forming fabric, the wire, formerly woven from metal strands, is now most commonly woven from polyester monofilament in a pattern that ensures both a smooth forming surface and a sufficient permeability to allow much of the water to drain through and out of the furnish.
At the end of the forming section, the wet fibrous sheet is still largely water, yet is now substantial enough to be picked up by a press felt and carried through the press section.
The press felt itself is also a continuous fabric belt, although much different from a forming wire. Today, it is literally inaccurate to refer to these fabrics as "felt". Years ago, however, they were produced from wool and, after weaving, felted by traditional methods to produce a fabric having a smooth surface. Wool has fallen largely out of use, except for the production of certain very fine, specialized papers. Yet, just as the forming fabric is referred to as a "wire" even though it contains no metal, the term "felt" is still used widely, if not universally, to refer to the fabrics used in the press and dryer sections.
The press felt has the two-fold purpose of carrying the wet fibrous sheet through the press section and of absorbing water that is squeezed from the sheet. The squeezing action takes place in the presses, where the felt and sheet are compressed together such as, for example, by passing together through the narrow gap, or nip, between two closely adjacent press rolls. There, the felt absorbs water squeezed from the sheet by the compression and prevents rewet. Quite commonly, the wet fibrous sheet is carried through the press section between two press felts in a sandwich-like fashion both to prevent damage to the sheet by the press rolls and to enhance the degree of water removal.
At the end of the press section, the wet sheet continues on to the dryer section where the remaining water is removed by conducting it around each in a series of steam-heated cylinders. The fabrics, or dryer felts, serve to hold the wet sheet firmly against the cylinder, in order to maximize the degree of heat transfer and more efficiently evaporate the remaining water. At the end of the dryer section, the sheet, now paper, emerges as the final product.
One of the important concerns in any papermaking operation is to produce a quality paper at maximum efficiency. Costs associated with the dryer section, such as those required to produce steam for the dryer cylinders, can cut deeply into potential profits, especially in this era of rising energy costs. Much of the search for improved press designs and better press felts is driven by the need to make the amount of water to be removed from the sheet in the dryer section as small as possible. In this way, it is hoped, costs for fuel consumed in steam production will be reduced.
The present invention represents a different line of attack toward a solution of this basic problem. It involves incorporating certain elements, normally found only in a press section, into a dryer section as a way to achieve a more fuel-efficient operation.